1. Field of the Invention
The present invention generally relates to systems and methods for forming an image of a specimen. Certain embodiments relate to systems and methods that include forming an image of a specimen at an oblique viewing angle.
2. Description of the Related Art
Fabricating semiconductor devices such as logic and memory devices may typically include processing a specimen such as a semiconductor wafer using a number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that typically involves transferring a pattern to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes may include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a semiconductor wafer and then separated into individual semiconductor devices.
During each semiconductor device fabrication process, defects such as particulate contamination and pattern defects may be introduced into the semiconductor devices. Such defects may be isolated to a single semiconductor device on a semiconductor wafer containing several hundred semiconductor devices. For example, isolated defects may be caused by random events such as an unexpected increase in particulate contamination in a manufacturing environment or an unexpected increase in contamination in process chemicals that may be used in fabrication of the semiconductor devices. Alternatively, the defects may be repeated in each semiconductor device formed across an entire semiconductor wafer. In an example, repeated defects may be systematically caused by contamination or defects on a reticle. A reticle, or a mask, may be disposed above a semiconductor wafer and may have substantially transparent regions and substantially opaque regions that are arranged in a pattern that may be transferred to a resist on the semiconductor wafer. Therefore, contamination or defects on a reticle may also be reproduced in the pattern transferred to the resist and may undesirably affect the features of each semiconductor device formed across an entire semiconductor wafer in subsequent processing.
Defects on semiconductor wafers may typically be monitored manually by visual inspection, particularly in the lithography process because many defects generated during a lithography process may be visible to the naked eye. Such defects may include macro defects that may be caused by faulty processes during this step. Defects that may be visible to the human eye typically have a lateral dimension greater than or equal to approximately 100 xcexcm. Defects having a lateral dimension as small as approximately 10 xcexcm, however, may also be visible on unpatterned regions of a semiconductor wafer. An example of a visual inspection method is illustrated in U.S. Pat. No. 5,096,291 to Scott and is incorporated by reference as if fully set forth herein. Prior to the commercial availability of automated defect inspection systems such as the systems illustrated in U.S. Pat. Nos. 5,917,588 to Addiego and 6,020,957 to Rosengaus et al., which are incorporated by reference as if fully set forth herein, manual inspection was common. Manual inspection id still widely used by lithography engineers.
Automated inspection systems were developed to decrease the time required to inspect a wafer surface. Such inspection systems may typically include two major components such as an illumination system and a collection-detection system. An illumination system may include a light source such as a laser that may produce a beam of light and an apparatus for focusing and scanning the beam of light. Defects present on the surface may scatter the incident light. A detection system may detect the scattered light and may convert the detected light into electrical signals that may be measured, counted, and displayed on an oscilloscope or other monitor. Examples of such inspection systems are illustrated in U.S. Pat. No. 4,391,524 to Steigmeier et al., U.S. Pat. No. 4,441,124 to Heebner et al., U.S. Pat. No. 4,614,427 to Koizumi et al., U.S. Pat. No. 4,889,998 to Hayano et al., and U.S. Pat. No. 5,317,380 to Allemand, all of which are incorporated by reference as if fully set forth herein.
An embodiment of the invention relates to a system configured to form an image of a specimen. The system may include a relay lens configured to form an intermediate image of light scattered by a specimen. As used herein, xe2x80x9clight scatteredxe2x80x9d by a specimen may include any light returned from a specimen such as reflected, scattered, and/or diffracted by the specimen. The relay lens may be positioned at an oblique viewing angle from an upper surface of the specimen. The oblique viewing angle may be, for example, approximately 30 degrees as measured from grazing incidence. The relay lens may be a unit magnification symmetrical relay lens. The relay lens may also be substantially telecentric in object and image space. Alternatively, the relay lens may be substantially non-telecentric to increase keystone distortion of the image of the specimen. In this manner, a controlled amount of blur may be produced at the area detector.
The system may also include a reflection grating positioned such that the intermediate image is imaged on the reflection grating. As used herein, a xe2x80x9cgratingxe2x80x9d may generally refer to an optical component that may have repeatable features, e.g., substantially parallel lines, intersecting lines, or concentric circles, formed upon a substrate that may reflect at least a portion of light striking the optical component. The reflection grating may be configured to reflect the intermediate image. The reflection grating may be positioned negative to the upper surface of the specimen, at the natural image plane. For example, if the oblique viewing angle is approximately 30 degrees as measured from grazing incidence, the reflection grating may be positioned at an angle of approximately xe2x88x9230 degrees to the optical axis of the relay lens. The reflection grating may be further positioned such that a grating surface of the reflection grating may be substantially perpendicular to the optical axis of an objective lens. The reflection grating may be substantially telecentric in object space.
The reflection grating may be a blazed reflection grating. A grating blaze angle of the reflection grating may be configured such that reflection of the intermediate image from each facet of the reflection grating may be substantially parallel to the optical axis of an objective lens. The reflection grating may include a grating pitch of approximately 10 lines per millimeter to approximately 30 lines per millimeter. Alternatively, the reflection grating may have a grating pitch configured to produce first order reflection of the intermediate image substantially parallel to the optical axis of the objective lens for a wavelength of the light.
In addition, the system may include an objective lens configured to focus the reflected intermediate image. The system may further include an area detector configured to produce a signal representative of the focused image. The area detector may include, for example, a time delay integration (xe2x80x9cTDIxe2x80x9d) camera. An image of the specimen may be formed from the produced signal. The image may be a high resolution rectilinear image of a portion of the specimen. The image may also be used to detect defects on the specimen.